Creative Developments (Cosmetics) Limited
Cosmetic Efficacy
Testing SPC 2002
John Woodruff
The phrase cosmetic testing elicits many different responses. To the public at large it conjures up a picture of cute fluffy mammals undergoing physical abuse; to the microbiologist it means challenge testing; the quality assurance department think of pH and viscosity measurements, activity levels and gas chromatograms. The formulating chemist thinks of all of these plus stability testing and to the demand of the 6th Amendment to the Cosmetics Directive 76/768/EEC, that where justified by the nature of the product, requires proof of effect.
Not only is it the marketing department that indulges in creative copy writing; critical reading of many patents may lead the sceptical to wonder if the formulating chemist knew in advance about “the surprising result” and claims for technical innovation that were to be made for the latest formulation. The need to prove claims for either the legal or the marketing and advertising departments has led to a proliferation of methods of substantiating such claims and of test laboratories able to undertake the necessary research.
Trying to provide proof of effect is
not new: the author attended the 2nd IFSCC Congress, London, in 1962
at which papers on such diverse subjects as measuring perspiration rates, oral
hygiene, skin oiliness, epidermal permeability and skin pH were presented.
There were also papers on the design of clinical trials and of standardising
salon tests. The same subjects can be heard at conferences today,
although the methods may have become more sophisticated.
A leading testing laboratory1 supplied a breakdown of the projects undertaken over the past year. Although providing proof of effect is an essential requirement of the 6th Amendment only 13% of the tests commissioned were for this and of those 70% were SPF and UVA testing. The majority were irritancy patch tests and human 21day repeat insult patch tests for cumulative skin irritation (HRIPTS). Home use covers household and baby care products other than toiletries and cosmetics.
Although providing the proof is very much secondary to safety testing it is none the less an essential aspect of cosmetic formulation. This feature will look at some typical claims for cosmetic skin care compositions and at a selection of methods and instruments available to provide proof of efficacy.
Typical Claims for Skin Care
Products
q Moisturises
q Improves skin elasticity
q Reduces fine lines & wrinkles
q Softens, smoothes & firms skin
q Delays the visible signs of ageing
q Deeply penetrates
q Restores natural pH balance
q Protects against free radicals
The four key measurements for determining the condition of the skin are moisture levels, transepidermal water loss, skin elasticity and cell turnover. The most readily perceived attribute is dry skin so it is not surprising that moisturising effect was one of the first aspects to be studied in detail. The claim “moisturises the skin” implies the moisture level in the epidermis will increase after a single application and will remain at an increased level for the rest of the day. In fact claims for 24 hour moisturising are not uncommon.
The Canadian Cosmetic Regulations define a skin moisturiser as a product applied to the skin to soften dry skin or maintain skin suppleness by reducing water loss or increasing the water content of the skin. The definition includes emollients and humectants. The CTFA does not define moisturisers but does define skin-conditioning agents (humectants) as cosmetic ingredients intended to increase the water content of the top layers of skin2 and a similar function is attributed to emollients and occlusive materials. The EU Cosmetic Regulations do not define moisturisers but define emollients as substances that are added to cosmetic substances to soften and smooth the skin.
Providing proof of effect frequently involves the use of human volunteers. In order to protect their rights the proposed study should be scrutinised by an ethics committee under the guidance of a medically qualified person. Assuming that the composition to be studied is deemed to be safe there are a variety of methods for providing proof of moisturising ability. A typical protocol for a subjective evaluation is to take fifty subjects in the age range 18 – 65 with visibly dry skin. They are studied for one week to form a base line assessment and then divided into groups according to their skin’s degree of dryness. Each group then uses the appropriate product for three weeks at home and according to instructions and then completes a questionnaire. A facial assessment is performed against a facial chart, subdividing the face into ten sections and each section is assessed for dryness. Hands are subdivided into six sections and dryness and surface roughness are scored. The results may be analysed statistically.
Using suitable instrumental techniques can enhance the study. A well-known instrument is the Corneometer™. The dielectric constant of water is 81 whereas for most other substances it is less than 7. The Corneometer uses a capacitance method to measure the change in dielectric constant after application of the test composition and this is translated into moisture level. The Nova Dermal Phase™ also uses capacitance measurements and the Skicon-100™ measures electrical conductance;
There is continuous diffusion of water from the body to the stratum corneum and to the environment. This is termed Transepidermal Water Loss (TEWL) and TEWL figures are often quoted in ingredient suppliers’ literature. The lower the TEWL the better the barrier function of the skin and the less natural moisture loss. The barrier function is readily disturbed by physical damage or chemical attack and such damage results in a change in the TEWL measurements. Ingredients that do not increase TEWL are most likely not to be irritants or to cause allergic skin responses.
TEWL may be measured using a closed chamber attached to the skin with a moisture probe reading the humidity in the chamber. Alternatively an open chamber may be used and two instruments are available. The Evaporimeter™ includes a probe for measuring temperature and relative humidity while the Tewameter™ measures the vapour pressure gradient within the chamber.
Typically a suitable number of subjects are selected that show a 2 – 3 fold increase in TEWL reading without suffering skin irritation following treatment with sodium lauryl sulfate (SLS) solution on the inside forearm. Symmetrically opposite sites are treated with standard SLS solution and then the test or control substances are applied twice daily. TEWL measurements and visual skin assessments are made before and after SLS treatment and daily thereafter over a set number of days. The rate of recovery to the pre-treatment levels are compared for test and control treatments and subjected to statistical analysis. TEWL measurements are affected by a number of variables including race, gender, the site of application and the emotional state of the test subject so correct experimental design is critical. It is important to undertake such studies in a controlled environment at standard temperatures and relative humidity and to give the subjects time to reach equilibrium before each assessment is made.
If a TEWL probe is held in direct contact with the skin, moisture is prevented from escaping and it builds up within the stratum corneum. This is a measure of water-holding capacity and is termed the moisture accumulation test or MAT. When MAT and TEWL are measured together new information can be obtained. In cases where the barrier is severely perturbed MAT and TEWL parallel with a high degree of correlation. In cases where there is dry skin or scaly dermatitis the measurements vary in opposite directions. This is because MAT reflects not only water movement but water holding capacity as well. The authors3 of a paper given at Stratum Corneum III hypothesised that the reduction in MAT in dry scaly skin is due to a reduction in natural moisturizing factor (NMF).
There are many other methods of measuring skin moisture levels and tests may also be made in-vitro on excised skin and reconstructed or model skin. For a comprehensive description of the major in-vitro study methods available for the measurement of skin properties with particular emphasis on product safety testing I recommend In-vitro testing models for claims substantiation by Rolf Mast & Stephen Rachui4.
Skin elasticity declines with age due to changes in the stratum corneum and in the underlying dermal structure. Claims to improve skin elasticity are almost as common as those for providing increased moisture. The simplest method of measurement uses a ballistometer. This is a pendulum that is dropped from a fixed height onto the skin surface. It measures the ability of the skin to absorb mechanical energy by analysing the bounce pattern displayed by a probe as it strikes the skin. The elastic components of the skin store the kinetic energy generated by the striking of the probe and cause the probe to rebound upon release. Smaller rebounds, with less kinetic energy, result when the probe strikes softer skin, bigger bounces occur from more elastic skin. Other methods cited include using uniaxial stretching whereby skin is twisted in one direction and the time taken to revert to its original resting position is measured; suction whereby twisting is replaced by the application of a vacuum, and indentation and shear disturbance propagation methods.
Dermatologists use their finger to investigate the skin by touching its surface softly or by pressing down to detect any presence of cutaneous changes, such as rough skin surface and loss of elasticity. An interesting attempt to replace the dermatologist’s finger and subjective assessment with a robot and objective measurements was described by Shu Sasai et al. His team developed a probe equipped with two independent sensors to measure different properties of skin hardness, i.e., a tactile vibration sensor and a displacement sensor. Two parameters are measured: one that reflects superficial firmness of the skin, which mainly correlates with the hydration state of the stratum corneum, and the other that reflects the firmness of deeper tissue.
Cell turnover is the shedding of the outer dead skin cells and their replacement by fresh ones. Failure to shed dead cells leads to dry itching skin, unsightly dry patches and to psoriasis. Cosmetic interest in improving cell turnover and exfoliation of the dead cells was stimulated by the use of retinol and of alpha hydroxy acids as skin peeling agents that are claimed to improve skin smoothness and moisture levels and to reduce the appearance of fine lines and wrinkles. A simple form of measurement is by tape stripping and the counting of dead cells with each successive strip. Similar techniques are used to measure the penetration of active agents and cosmetic excipients into the skin whereby the chemical is quantitatively identified by spectroscopy on each successive strip.
Skin smoothness is usually measured on a silicone elastomer replica using a mechanical stylus instrument, laser profilometry or a shadowing method. The Dermatest™ UB 16 optical measuring system is a semi-conductor laser with a measuring point diameter of 1 µm. The beam is focused onto the surface of the skin impression and reflected from there. Two program-controlled step motors move the measuring bench with the silicon impression under the beam. A half-silvered mirror guides the reflected beam through a prism onto two photodiodes. These register every defocus of the measuring point caused by the surface profile, and activate the electronic adjustment of the objective lens until the beam is refocused on the surface. At the same time, the position of the lens is measured and stored as a digital value. With appropriate selection of the point density for the x and the y-axis, the computer program generates a true-to-life, three-dimensional image of the skin relief on a colour monitor6.
After washing with soap the pH of normal healthy skin will rise to approximately 8, it will regain its natural pH of about 5.5 within two to three hours. Products abound that claim to restore the skin’s natural pH balance somewhat more quickly. This can be measured using a specially designed pH electrode. Protection against free radicals is not so simple to prove: a common in-vitro way of assessing antioxidant efficacy is to monitor the oxygen consumption of the system that one wishes to protect in the presence and absence of each antioxidant that is to be tested using a Warburg manometer. A more advanced instrumental technique uses an instrument called Photochem™ that utilises photochemiluminescence reactions. Free radicals are generated within the system that react with a photosensitiser to emit light. In the presence of free radical scavengers the intensity of the photochemiluminescence is attenuated as a function of concentration and the anti-radical properties of the substances can be quantified.
There are many other properties of skin that can be measured and many different ways of measuring them. The suspicion is that a method can always be found to provide the results required, and where the method is ambiguous statistics come into their own!
1. Sequani Limited, Ledbury, Herefordshire; private communication.
2. CTFA International Cosmetic Ingredient Dictionary & Handbook, 9th Edition, 2002
3. Wickett, R, Tolia, G., Visscher, M., The effect of extraction and NMF treatment on the water handling capability of stratum corneum as measured by TEWL and MAT; Stratum Corneum III, Basel, September, 2001
4. Aust L., Cosmetic Claims Substantiation, pp 171 – 209, Marcel Dekkar Inc. New York, 1997
5. Shu
Sasai et al; Palpation of the skin with a robot finger: an attempt to
measure skin stiffness with a probe loaded with a newly developed tactile
vibration sensor and displacement sensor. Skin
Research and Technology 1999; 5: 237-246.
6.
Breuer M., Voss W., Proving For Efficacy: Laser
Profilometry; Dermatest GmbH
Further reading:
Bioengineering of the Skin: Skin Biomechanics; September 2001 Peter Elsner, Enzo Berardesca, Klaus P. Wilhelm; CRC Press, 2002
Aust L., Cosmetic Claims Substantiation, Marcel Dekkar Inc. New York, 1997
Instruments mentioned in the text
Corneometer™; Courage & Khazaka, Cologne, Germany
Evaporimeter™; Servomed, Stockholm, Sweden
Tewameter™; Courage & Khazaka, Cologne, Germany
NOVA Dermal Phase Meter™; NOVA Technology, Portsmouth, NH.
Photochem™; Analytik Jena AG, Konrad-Zuse-Str. 1 • 07745 Jena • Germany
A Selection of Skin Product
Efficacy Testing Laboratories
Cardiff Biometrics Ltd
Cardiff, Wales, UK
Phone: +44 (0)29 2041 0074
Email: info@cardiff-biometrics.co.uk
Courage & Khazaka Electronic GmbH
Mathias-Brüggen Strasse 91,
D-50829 Köln, Germany.
Tel: +49 221 95 64 99 25
Fax: +49 221 95 64 99 20
Email: courage@t-online.de
Miss Diana Khazaka
Cutest Systems Ltd.,
178/174 Whitchurch Road, Heath, Cardiff,
CF4 3NB, Wales, UK
Tel: +44 (0)1222 747426
Fax: +44 (0)1222 614688
Email: peter.dykes@cutest co.uk
Dr. Peter Dykes and Dr. Tony Pierce
Dermatest GmbH
Birkenweg 4
Münster 48155
Germany
Tel:
+49 251 311 910
Fax: +49 251 311 1479
Email: dr.voss@t-online.de
Dr Voss
Globecrown Services Ltd.
Globecrown House
32 High Street, Maldon
Essex CM9 5PN, UK
Tel: +44 (0)1621 842 460
Fax: +44 (0)1621 842472
E-mail: globe.crown@ukonline.co.uk
Lynda Noakes
Inveresk Research International Ltd.,
East Lothian EH33 2NE
Scotland
Tel: +44 (0)1875 614545
Fax: +44 (0)1875 614555
PPD Pharmaco
Chelmsford Clinical Trials Unit Ltd.,
Townfield House,
30-33 Townfield Street,
Chelmsford, Essex
CM1 1QL, UK
Tel: +44 (0)1245 252878
Fax: +44 (0)1245 490451
Email: grant.barnett@europe.ppdi.com
Sequani Limited
Bromyard Road
Ledbury, Herefordshire
HR8 1LH, UK
Tel: +44 (0)1531 634121
Fax: +44 (0)1531 631554
Skin Research Centre (UK) Ltd.
169 Petersfield Avenue,
Staines, Middlesex
TW18 1DH, UK
Tel: +44 (0)1784 452322
Margaret Batt